Traditionally, a provider of professional services (e.g., a provider of legal services such as an attorney or a law firm, a provider of accounting services such as an accountant or an accounting firm, architects, public relations professionals, etc.) dictates the amount of time spent and service fees associated with professional services rendered to a client. They usually bill at a certain increment on the billable hour model.
For example, when an attorney performs legal services for a client, the amount of legal service fees the client is billed may be based on multiple factors, such as the total amount of time spent by the attorney consulting with the client and then preparing documents, attending meetings or conducting other work, the complexity of the legal services performed, the hourly rate of the attorney, etc. The client may have little or no input with regards to controlling the costs or fees associated with the legal services being performed.
Alternatively, sometimes professionals quote flat fees for specific services. However, again, this effort is simply an attempt by the professional to model the projected time they are going to dedicate to a project. It is inaccurate and completely within the professional's control. The client is not able to define the amount of time they want the professional to dedicate to the project nor even the amount of time that the professional is consulting with the client.